A cellular communication system is a complex network of systems and elements. A minimum number of elements include 1) a radio link to the subscriber units (cellular telephones or mobiles) which is usually provided by at least one and typically several base transceiver stations (BTS), 2) communication links between the base transceiver stations, 3) a controller, typically one or more base station controllers or centralized base station controllers (BSC/CBSC), to control communication between and to manage the operation and interaction of the base transceiver stations, 4) a call controller or switch, typically a mobile switching center (MSC), for routing calls within the system, and 5) a link to the land line or public switched telephone system (PSTN) which is usually also provided by the MSC.
One aspect of designing a wireless communication system requires selecting geographic locations of base stations and associated service areas. Several requirements of the design include ensuring that certain radio frequency channels, such as pilot, page and sync channels, are present in each service area, while further ensuring interference with other service areas is minimized.
Typically, pilot-channel powers at each base station are manually adjusted, while page- and sync- channel powers are automatically scaled to the manually adjusted pilot-channel power levels. Traffic-channel power limits may also be automatically scaled to the adjusted pilot-channel power levels. For simplicity, pilot-, page- and sync-channel powers are referred to collectively herein as pilot-channel powers, although it is contemplated that the term pilot-channel power may apply to one or more such channels or to any similar type of channel. Although it is possible to select a specific pilot-channel power, prediction of the actual signal propagation pattern of the pilot-channel may be difficult where the signal may be reflected in unanticipated directions, resulting in insufficient signal strength in a base station's designated service area and/or unacceptable levels of interference in neighboring service areas. For example, setting one or more pilot-channel powers too high may cause too many pilot-channel signals to be present at various locations throughout the system, and may hinder a mobile's ability to select the best pilot-channel from a pathloss perspective. This may have a particularly detrimental effect on soft handoff performance of the system. It may also cause the signal-to-noise ratio from all of the pilot-channel powers at a particular location to be unacceptable.
As will be appreciated, manual assignment of pilot-channel powers is time consuming and expensive in that assignments must be made, field tested and revised repeatedly to obtain complete coverage and acceptable interference levels for each service area in the communication system. An automated method of setting pilot-channel powers is described in commonly-assigned U.S. patent application Ser. No. 08/884,965, filed Jun. 30, 1997, entitled "Method for Automatically Selecting Channel Powers in a Wireless Communication System" the disclosure of which is hereby expressly incorporated herein by reference. The automated method disclosed in the aforementioned United States patent application is further useful in making subsequent pilot-channel power settings and/or revisions to the existing pilot-channel power settings. However, each set of pilot-channel power settings remains "static" from a system operation standpoint. That is, in real-time, during operation of the wireless communication system, the current set of pilot-channel power settings remains unchanged. Changes to the active set of pilot-channel powers occurs infrequently, and then usually only as a result of a significant change in the wireless communication system, for example, cell addition, topology change, etc.
Thus, even with the improvement in pilot-channel power assignment offered in U.S. patent application Ser. No. 08/884,965, the static set of pilot-channel powers is a compromise and cannot simultaneously make all areas of the system "good" during system operation. The static set seeks the best condition over as many areas at once simultaneously, but does not account for current areas of high and low traffic density, i.e., where the mobiles are located, at any given point in time. In addition, pilot-channel power settings based on a single distribution or average of distributions for a particular cellular system design, i.e., cell locations, sector antenna characteristics and covered terrain, may leave some important areas not properly served as the traffic moves around and changes the noise at each base (reverse link) and at each location (forward link) in the system. The phrase "not properly served" is used to refer to the instance in which the service area is either under covered or over covered. An under covered service area lacks pilot-channels of sufficient number and power to properly service the mobiles operating in the coverage area. An over covered service area may have so many pilot-channels impinging on a given area as to require the mobile to switch more rapidly than the control system can measure and message just to remain connected to a good one. An over covered service area may also have too many pilot-channels of significant power where no one is sufficient due to the presence of the others and other noise sources. That is, no one pilot-channel stands out for the mobile to select. The result of poorly adjusted pilot-channel power is dropped calls, failed handoffs, system unavailability and system underutilization.
Thus, there is a need for a method of adjusting channel powers within a wireless communication system during operation of the wireless communication system.